Electrophotography in which the electrostatic latent image formed on the latent image bearing member such as a photosensitive member is developed with charged (toner) and an image is formed has become widespread. As the electrophotography of this kind, for example, a dry developing method directly using powdery toner and a wet developing method (liquid developing system) using the liquid developer in which the toner is dispersed in a liquid exist. Of these, in the liquid developing system, the toner is dispersed in the medium (carrier) liquid, and therefore, image formation can be carried out by controlling particles with a particle size in a submicron order, and the liquid developing system is a promising developing method in terms of high image quality and high definition.
Further, the liquid developer is advantageous in handling such that for example, the toner is not scattered, and therefore, image formation at high speed is enabled. Therefore, the liquid developing system is frequently used in an image forming apparatus for professionals, such as light printing and graphic arts, in which the high image quality is required over a long term. For that reason, the liquid developing system is required not only that states of respective functional component parts contributing to the image formation are properly maintained but also that when the states are out of required performances, the functional component parts are appropriately replaced with those satisfying the performances.
In the liquid developing system, a developing process in which the electrostatic latent image drawn on the latent image bearing member (hereinafter, referred to as a photosensitive drum) is visualized with the toner is a mostupstream process for forming a toner image. Therefore, it is very important for properly maintaining the toner image on the photosensitive drum with a high image quality that the performance of a developer carrying member (hereinafter, referred to as a developing roller) performing the function of feeding the toner onto the photosensitive drum, i.e., that the developing roller is used within a lifetime range thereof.
The developing roller used in the liquid developing system is formed in general by using a metal shaft as a core and by providing a surface layer of an elastic member made of a polymer, a rubber material or the like of which electrical conductivity is adjusted, on a peripheral surface of the core. The developing roller reaches an end of a lifetime by deteriorations in a mechanical characteristic such as a surface property and in an electrical characteristic such as an electroconductive property, but as regards lifetime extension thereof, reduction of the deterioration in mechanical characteristic of the developing roller is relatively easier than reduction of the deterioration in electrical characteristic of the developing roller.
As an example of the deterioration in mechanical property, the following example exists. For example, there is a case that an application roller with surface unevenness is used for carrying the developer on a surface of the developing roller, but when the application roller directly contacts the developing roller, there is a liability that the surface of the developing roller is damaged, so that the deterioration in mechanical characteristic is invited. In order to reduce such a deterioration in mechanical property, for example, an image forming apparatus in which a liquid developer applied onto a surface of an application roller is applied onto the surface of a developing roller through an intermediary roller has been developed (Japanese Laid-Open Patent Application (JP-A) 2002-287513 and JP-A 2003-156938). According to this image forming apparatus, by providing the intermediary roller between the application roller and the developing roller, mechanical abrasion and damage due to unevenness of the application roller at the surface of the developing roller are reduced, so that lifetime extension of the developing roller can be realized.
On the other hand, as the electrical characteristic of the developing roller, in general, volume resistivity of the developing roller is optimized by dispersing and mixing an ion conductive agent into an elastic polymer constituting a surface layer. Before use of the developing roller, the ion conductive agent is uniformly dispersed in the surface layer, but during an image forming operation, different voltages are applied to the developing roller and the photosensitive drum or the like disposed at a periphery of the developing roller, and therefore, the dispersion of the ion conductive agent gradually generates localization. Therefore, volume resistivity of the surface layer of the developing roller increases with use. With the increase in volume resistivity of the surface layer of the developing roller, a shearing voltage applied to the surface layer of the developing roller increases in a gap between the developing roller and the photosensitive drum or the like, and therefore, a voltage applied to the develop becomes small compared with a desired value. Accordingly, when the volume resistivity of the surface layer of the developing roller increases up to the extent that the voltage applied to the developer is insufficient for image formation, the developing roller reaches the end of the lifetime. It is difficult to suppress the localization of the ion conductive agent, and therefore, the electrical characteristic of the surface layer of the developing roller, i.e., the electroconductive property constitutes a rate-determining factor of the lifetime. For this reason, in the image forming apparatus using the liquid developing system, exchange timing, i.e., the lifetime of the developing roller is determined depending on the number of image formation sheets which is a use frequency of the developing roller in some cases.
However, in the image forming apparatuses of the above-described JP-A 2003-156938 and JP-A 2003-156938, the exchange timing of the developing roller is determined depending on the number of image formation sheets by the developing roller. Here, the electroconductive property of the developing roller surface layer which constitutes the rate-determining factor is influenced by a using method such as an image ratio in addition to the number of image formation sheets of the developing roller, and in addition, there is also a difference among individuals of the developing rollers, and therefore, an actual lifetime of the developing roller cannot be discriminated with high accuracy only by a simple number of image formation sheets. For this reason, in order to determine the lifetime of the developing roller depending on the number of image formation sheets simply, in consideration of other factors which have the influence on the lifetime of the developing roller, there is a need to set the number of image formation sheets, which is the lifetime, at a small value. By this, the case where the developing roller is exchanged although the developing roller does not actually reach the end of the lifetime occurs, and an exchange frequency increases more than necessary, so that there is a liability that a running cost of the image forming apparatus is increased and that a lowering in productivity is invited by an increase in exchange operation.
The present invention aims at providing an image forming apparatus capable of discriminating a lifetime of a developing roller individually with high accuracy in the image forming apparatus using a liquid developing system.